The present invention relates to a compensating device for raster distortion of a cathode-ray tube apparatus that is used in a television set or a computer display.
Recently, a screen panel of a cathode-ray tube apparatus is required to be substantially flat. One of the problems involved with the flatness of the screen panel is a raster distortion. Though the shape of the raster on the screen must be an exact rectangle for displaying a high quality image, in practice it can be a distorted shape such as a trapezoid or a pincushion due to an influence of a deflection yoke, an electron gun or other members of the cathode-ray tube apparatus. Compensation for such raster distortion is usually performed by controlling the current supplied to a deflection coil using a compensation circuit provided to the display set, as described in Japanese laid-open patent application (Tokukai-Hei) 3-6176 or 6-31272, for example.
Even if an exact rectangular shape of the raster is obtained after such compensation for a raster distortion, the raster sometimes still has an inner distortion as shown in FIGS. 6A and 6B, for example. In these Figures, the left and right sides as well as the upper and lower sides of the raster are straight and parallel, but the middle portions 20' and 20" of the raster are distorted to a pincushion shape. This type of distortion is called "Inner pincushion distortion" hereinafter. FIG. 6A shows a horizontal inner pincushion distortion where the middle portion 20' including a center vertical axis 12 is distorted to a pincushion shape. FIG. 6B shows a vertical inner pincushion distortion where the middle portion 20" including a center horizontal axis 11 is distorted to a pincushion shape. A testing method of such inner pincushion distortions is described in "Testing Methods for Cathode-Ray Tubes with Deflection Yoke" as a Standard of Electronic Industries Association of Japan, ED-2101B revised on February, 1995, Section 7.8.
The inner pincushion distortion mentioned above can be compensated by a method disclosed in Japanese laid-open patent application, Tokukai-Hei 4-188543 or Jitsukai-Sho 63-108151. In this method, an inner magnetic shield member in a glass bulb of the cathode-ray tube is cut by a circular curve at upper and lower sides or left and right sides of the rectangular opening that is near to an electron gun such that the end of the cut side retreats at the middle portion. Thus, a vertical or a horizontal magnetic field is distorted, so that the inner pincushion distortion is compensated.
However, the above mentioned method for compensating the inner pincushion distortion has some disadvantages. A shield effect by the inner magnetic shield member may be weakened due to the change in shape of the shield member. Mechanical strength also may be weakened due to the cutouts of the shield member.
In addition, a cost for the compensation of the raster distortion may be increased since different means are required for the compensation for the different raster distortions such as the trapezoid distortion and the inner pincushion distortion. A compensation circuit that is added to the display set for the raster distortion such as the trapezoid distortion is complicated and expensive.
A purpose of the present invention is to provide a compensation device for a raster distortion, which can compensate the raster distortion including the inner pincushion distortion at a low cost and easily.